1. Field
The invention relates to radio channel emulation.
2. Description of the Related Art
Communication between a DUT (Device Under Test) and a base station of a radio system can be tested using a wired connection. When a radio frequency signal is transmitted from a transmitter to a receiver, the signal propagates in a radio channel along one or more paths having different signal phases and amplitudes, which causes fadings of different duration and strength in the signal. In addition, noise and interference caused by other transmitters interfere with the radio connection.
Antennas used in base stations of real radio systems are often arrays having a plurality of antenna elements. When an antenna has N antenna elements where N is an integer larger than 1, they may be arranged in a desired two or three dimensional form of i×j or i×j×k antenna elements where i, j and k are positive integers. The base station may use all or a selected number P of antenna elements, where P is an integer equal to or smaller than N. A signal to the antenna is split to the desired number P of antenna elements and the antenna element signals are then phase shifted and power controlled in order to form a desired beam with the antenna.
A transmitter and a receiver can be tested either in real circumstances or by using a radio channel simulator simulating real circumstances. Since tests performed in real circumstances are typically cumbersome, subject to uncontrollable phenomena and depend on the test environment, a device simulating a radio channel is used to simulate a radio channel of the desired kind. In a digital radio channel simulator, a channel is usually modeled with a FIR filter (Finite Impulse Response filter), which generates convolution between the channel model and an applied signal by weighting the signal, delayed by different delays, with channel coefficients, i.e. tap coefficients, and by summing the weighted signal components. The channel coefficients are modified to correspond to the behaviour of a real channel.
In order to perform radio channel emulation with the antenna array, each signal from the base station to the antenna is split into P antenna element signals each directed to one antenna element and the antenna element signals are phase shifted and power controlled by phase shifters and attenuators. Every signal is then separately connected to the radio channel emulator. Since the number M of desired antenna elements may be high, for example tens or hundreds, the radio channel emulator must have a lot of input ports for the signals from the antenna elements and it must be able to emulate each antenna signal input to its port.
However, the emulation is cumbersome with a splitter, a large number of phase shifting and power control elements. During different radio channel emulations the number of antenna elements, the phase shifts and the weights may also differ. Moreover, the requirement of a large number of the input ports and emulation channels may be challenging or even impossible for the emulator. Hence, there is a need for a better solution to perform radio channel emulation with an antenna array of plurality of antenna elements.